PROJECT SUMMARY/ABSTRACT Latent HIV reservoirs, primarily comprised of HIV-infected and long-lived subpopulations of CD4+ resting memory T cells are established during the earliest stage of infection. While currently available antiretroviral therapies (ART) can reduce the level of HIV in blood to an undetectable level, they cannot eliminate the latent reservoir, thereby imposing a major obstacle to curing the infection. A number of latency reversal agents (LRAs) has shown activity in vitro, but all have little or no impact on the latent reservoir in clinical trials to date. Because increasing the doses of LRAs is prohibitive in their current forms due to the potential for increased systemic toxicity, alternative strategies for the specific targeting and activation of the latent HIV reservoir are needed. We therefore propose to harness the exquisite specificity of monoclonal antibodies (mAbs) and to further increase their specificity through the construction of bispecific antibodies for targeting the HIV latent reservoir. We will engineer a panel of bispecific antibodies capable of targeting the narrow subsets of CD4+ resting memory T cells that have been characterized as the likely HIV reservoir cells in blood and tissues. We will then conjugate a panel of LRAs to each of the promising bispecific antibodies to deliver such agents preferentially to the latently infected cells. To evaluate the activity of all of our engineered bispecific antibodies and antibody-drug conjugates (ADCs), we will take an iterative approach to assess binding affinity and avidity, selectivity of LRA delivery, and HIV activation from the latent reservoir in vitro or ex vivo. The ADCs with the most promising in vitro or ex vivo properties will be further evaluated for their impact on the latent reservoir in vivo using a humanized mouse model of HIV infection and treatment. The underlying hypothesis of the proposed studies is that the use of bispecific antibodies to concentrate LRAs in cell populations harboring latent HIV, while minimizing the exposure to cells that are virus free, could markedly increase the therapeutic index of LRAs by several orders of magnitude. We believe that our proposal offers a promising and novel strategy for highly specific and potent activation of HIV reservoir cells, and it is our belief that one or several of our engineered antibody drug conjugates could become a key component in a multi-pronged approach to eliminating the latent reservoir and curing HIV-1 infection.